Electrochemical storage batteries, and in particular, lead sulfuric acid storage batteries are ubiquitous in automotive applications. These batteries have electrochemical cells developing about 2.15 Volts each. A generic lead acid battery cell has a positive electrode, a negative electrode, and aqueous sulfuric acid as part of the electrolyte. The electrodes are held in parallel and electrically isolated by a porous separator to allow free movement of charged ions. Generally, six of these cells are connected in series to produce the 12 Volts common in automobile systems.
Lead acid battery cells are quite unique because the electrolyte actively participates in the energy storage and release process, as represented schematically in Equations 1, 2, 3, and 4 below:                     Electrolyte        ⁢                  xe2x80x83                ⁢                                                                              xe2x80x83                                ⁢                                                      H                    2                                    ⁢                  S                  ⁢                                      xe2x80x83                                    ⁢                                      O                    4                                                                                      ⇌                                                                        H                  +                                +                                  HSO                  4                  -                                                                                        Equation        ⁢                  xe2x80x83                ⁢        1                                Negative        ⁢                  xe2x80x83                ⁢        Electrode        ⁢                  xe2x80x83                ⁢                                                                              Pb                                      (                    metal                    )                                                  +                                  HSO                  4                  -                                                                                                      ⇌                  Charge                                Discharge                                                                                      PbSO                  4                                +                                  H                  +                                +                                  2                  ⁢                                      e                    -                                                                                                          Equation        ⁢                  xe2x80x83                ⁢        2                                Positive        ⁢                  xe2x80x83                ⁢        Electrode        ⁢                  xe2x80x83                ⁢                                                                              PbO                  2                                +                                  3                  ⁢                                      H                    -                                                  +                                  H                  ⁢                                      xe2x80x83                                    ⁢                  S                  ⁢                                      xe2x80x83                                    ⁢                                      O                    4                    -                                                  +                                  2                  ⁢                                      e                    -                                                                                                                        ⇌                  Charge                                Discharge                                                                                      PbSO                  4                                +                                  2                  ⁢                                      xe2x80x83                                    ⁢                                      H                    2                                    ⁢                                      xe2x80x83                                    ⁢                  O                                                                                        Equation        ⁢                  xe2x80x83                ⁢        3                                Total        ⁢                  xe2x80x83                ⁢        Reaction        ⁢                  xe2x80x83                ⁢                                                                              Pb                                      (                    metal                    )                                                  +                                  PbO                  2                                +                                  2                  ⁢                                      H                    2                                    ⁢                  S                  ⁢                                      xe2x80x83                                    ⁢                                      O                    4                                                                                                                        ⇌                  Charge                                Discharge                                                                                      2                  ⁢                                      xe2x80x83                                    ⁢                                      PbSO                    4                                                  +                                  2                  ⁢                                      H                    2                                    ⁢                  O                                                                                        Equation        ⁢                  xe2x80x83                ⁢        4            
Within the electrochemical cell, lead metal (Pb) supplied by the negative electrode reacts with the ionized sulfuric acid electrolyte to form various lead sulfates, generally represented herein (Equation 2) as PbSO4. Charging of the battery cell via an external electrical current converts these sulfates into the positive active mass (hereinafter PAM), including electrically conductive lead dioxide (PbO2 of Equation 3). In particular, charging of the cell converts the PbSO4 into PAM, discharge releases the stored electrical potential when the PAM is converted back into PbSO4.
It is important to battery performance that the PAM be in physical and electrical contact with the positive electrode. Accordingly, the PAM must be supported by, adhered and/or attached to, and in electrical communication with the positive electrode for the battery to function properly. Separation of the PAM from the positive electrode results in poor battery performance and ultimately in battery failure, which is defined herein as a battery no longer suitable for its intended purpose. Battery performance is affected by the materials from which the positive electrode is formed, the physical configuration of the positive electrode, and the method by which a xe2x80x9cgreenxe2x80x9d metal grid (i.e., a bare metal grid or core) is converted into the positive electrode.
Green battery grids are typically lead alloys formed into a grid structure by a variety of methods. Historically, the processes by which green grids are made (i.e., transformed) into positive grid electrodes have a number of common steps including: pasting, steaming, curing, pickling and/or forming.
In pasting, a paste including water, sulfuric acid, lead and lead oxides is applied to the grid surface. The pasted grid may then be steamed (i.e., 100xc2x0 C. and 100% humidity) to facilitate crystal growth within the paste. The grid is then cured at controlled temperature and humidity conditions to xe2x80x9csetxe2x80x9d the paste, wherein the paste is chemically transformed into sulfates, hydroxides, carbonates, and other lead compounds through a series of complex hydration reactions requiring the presence of water. These reactions take place within the paste itself, and between the paste and the grid metal. Importantly, curing produces a xe2x80x9ccorrosion layerxe2x80x9d at the interface between the grid and the paste, which provides physical and electrical communication between the PAM and the positive grid electrode, as well as protection of the grid from attack by the electrolyte.
Once cured, the grids are assembled into a battery package and charging electrolyte added. By allowing the battery package to stand for a period of time, the grids are xe2x80x9cpickledxe2x80x9d. An external electric current is then passed through the cells in the forming step, wherein a majority of the paste is converted into PAM. The charging electrolyte is then removed and the battery is filled with shipping electrolyte to render the battery ready for use.
Phenomena that have a negative effect on battery performance include fracture lines that form due to stress introduced into the PAM layer as it accumulates on the positive electrode during charging. Also, when the lead in the positive electrode grid reacts with water as shown in Equation 5 below:
Pb(metal)+H2Oxe2x86x92PbO+2H++2exe2x88x92xe2x80x83xe2x80x83Equation 5
di-electric (i.e., non-conductive) lead oxide (PbO) is formed on the surface of the grid, which renders the affected portion non-conductive, and impacts support of the PAM layer. The metal grid can also react with the sulfuric acid electrolyte to form pits through pores, cracks, or holes in the corrosion layer, and from non-uniformities in the chemical composition and microstructure of the layer. Pits destroy the interface between the grid and the PAM, break electrical communication, and destroy physical contact (i.e., support) between the positive electrode grid and the PAM layer. Accordingly, the afore mentioned phenomena, either alone or in combination, result in decreased battery performance (i.e., the cell losing its capacity to transfer and store electrical energy), which eventually leads to battery failure. While these phenomena are significant at room temperature, they become even more significant at higher operational temperatures.
The rates at which the afore mentioned chemical processes proceed is proportional to temperature. The higher the temperature, the faster the reaction rate (i.e., the higher the temperature, the more PAM that forms, the more PbO that forms, and the more pitting that takes place). Positive grid corrosion becomes particularly significant under xe2x80x9chigh temperaturexe2x80x9d conditions (defined herein as above 50xc2x0 C.), which have become common in automotive applications as xe2x80x9cunder hood temperaturesxe2x80x9d rise due to automotive design trends and space limitations.
Accordingly, it is desirable to reduce or substantially eliminate high temperature effects on positive battery electrodes of lead acid batteries. In particular, to provide a longer useful-lifetime of the battery, preferably utilizing materials that provide an economic incentive in doing so.
Provided herein is an electrode including or having a noble-metal free grid containing lead, wherein the grid has an essentially PbO free PbO2 coating covering all, or essentially all of the surface of the grid. Also disclosed is a method of forming an electrode that includes applying an essentially PbO free PbO2 coating to the surface of a noble-metal free grid containing lead, wherein the coating covers all, or essentially all of the surface of the grid.
Furthermore a method of forming an electrode is disclosed including electrolytically depositing an essentially PbO free PbO2 coating onto the surface of a noble-metal free grid containing lead, tin and calcium, wherein the essentially PbO free PbO2 coating has a thickness not less than 5 microns and not more than 500 microns, and wherein the essentially PbO free PbO2 coating covers all, or essentially all of the surface of the grid; applying a paste having lead and lead oxide to the surface of the coated grid to produce a pasted grid; optionally contacting the pasted grid with steam for at least about one hour to produce a steamed grid; curing the steamed grid at about 50 percent humidity and about 55xc2x0 C. for at least 24 hours to produce a cured grid; followed by contacting the grid with aqueous sulfuric acid prior to passing an external electric current of sufficient voltage and amperage through the cured grid for a sufficient period of time to convert at least a portion of the applied paste into lead dioxide.
Also disclosed herein is a battery having a noble-metal free grid containing lead, wherein the grid has an essentially PbO free PbO2 coating covering all, or essentially all of the surface of the grid. A method of making a battery is also disclosed including applying an essentially PbO free PbO2 coating to the surface of a noble-metal free grid containing lead, wherein the coating covers all, or essentially all of the surface of the grid.
In addition, disclosed herein is a method of making a battery including electrolytically depositing an essentially PbO free PbO2 coating onto the surface of a noble-metal free grid containing lead, tin and calcium, wherein the essentially PbO free PbO2 coating has a thickness not less than 5 microns and not more than 500 microns, and wherein the essentially PbO free PbO2 coating covers all, or essentially all of the surface of said grid; then applying a paste containing lead and lead oxide to the surface of the coated grid to produce a pasted grid; optionally contacting the pasted grid with steam for at least about one hour to produce a steamed grid; and then curing the steamed grid at about 50 percent humidity and about 55xc2x0 C. for at least about 24 hours to produce a cured grid; followed by contacting the cured grid with aqueous sulfuric acid prior to passing an external electric current of sufficient voltage and amperage through the cured grid for a sufficient period of time to convert at least a portion of the applied paste into lead dioxide.